This invention relates to an improvement in an electrically heated transparency such as may be employed in a vehicle to provide defrosting, deicing, or defoggng capability. In particular, the improvement is in the means to detect discontinuities in the electric circuit in the transparency.
It has been known to pass electric current through a transparent conductive coating on a transparency in order to raise the temperature of the transparency. Generally, a source of electrical potential is connected to the conductive coating by way of a pair of bus bars along opposite sides of the area of the transparency to be heated. The bus bars have low resistivity relative to the coating and are intended to distribute the current evenly over the area to be heated. The bus bars may be comprised of metallic foil strips, but in the case of glass transparencies they preferably are comprised of a metallic-ceramic frit material fused onto a surface of the transparency. A typical arangement includes bus bars configured as substantially parallel stripes on opposite sides of the heated area, with electrical leads attached to each bus bar and extending away from the opposite edges of the transparency as shown in U.S. Pat. Nos. 4,323,726 (Crias et al.) and 4,668,270 (Ramus). Locating the leads on the same side of the transparency and preferably closely adjacent to each other is advantageous for the sake of easier installation of the transparency in the vehicle and simplifying the connection with the electrical power source. Therefore, U.S. Pat. Nos. 3,895,213 (Levin) and 4,543,466 (Ramus) provide an extension of one of the bus bars around an end of the transparency so that connections to both bus bars can be made in one relatively compact area.
A crack in a heated transparency can alter the electric heating circuit in ways that can cause further damage to the transparency or have other undesirable effects. A discontinuity in the coating extending with a transverse component to the direction of current flow will increase the overall resistance of the heated area, with the result that power output increases in the unaffected areas. Not only will the heating be ineffective in the damaged area, but also the increased power in the remainder of transparency can raise temperatures to such an extent that the transparency may be thermally damaged. Excessive temperatures can extend propagation of a crack in glass or melt a plastic ply. A break in a bus bar, can radically concentrate the electric power in a smal area, depending upon the location of the break. Because of the relatively large amount of current flowing along the bus bars, a defect such as a partial break that increases the resistance of a bus bar is particularly prone to cause localized overheating in the region near the defect. This can occur at any location along the bus bars, but it is particularly serious at locations where a bus bar is carrying the full current or a major portion of the current, such as in an extension leading to a remote bus bar. At high voltage locations, arcing across an open gap in the conductive material can also occur detrimentally. Although arcing is most likely to occur at a bus bar break, it can also occur across a discontinuity in the conductive coating. Another site for potential unbalanced heating or arcing is at the junction of the bus bars with the conductive coating, where the contact may be uneven or separation may occur. Because of the additional harm that overheating or arcing can cause in the transparency when minor damage occurs, it is considered desirable to provide means to detect such an occurrence so as to trigger an alarm device or to automatically remove electrical power from the heating system.
One approach that has been proposed for detecting bus bar breaks in a heated transparency employs a thin electroconductive voltage sensor lead applied to the transparency along with the bus bars. The sensor lead parallel the extension of the upper bus bar along one side of a windshield and contacts the bus bar system at the upper corner where the upper bus bar and the extension meet. External circuitry is provided to detect a voltage change along the extension evidencing a discontinuity in the extension. This approach is limited to detecting breaks in only the extension portion of the bus bars, and although breaks there may have serious consequences, it would be desirable to detect breaks at other locations as well, including the entire bus bar system, the coating, and the contact of the bus bars with the coating.
Additionally, arrangements that rely on detecting voltage changes are susceptible to false alarms due to fluctuations in the applied voltage due to varying loads on the power supply by other accessories. Accommodating the detection system to these fluctuations renders the system less sensitive to discontinuities in the heating circuit. It would be desirable to provide a detection system that is not affected by extraneous voltage fluctuations.